Computer is freezing.

Ok, I just built this machine for my birthday. It is: AMD XP 3000 w/ 400 FSB. ABIT KV-7 mobo. Seagate baracuda 120 gig Sata HD. ATI 9600 pro 128 mb. And a Modem Blaster modem. I also have a Sony 16x dvd-rom and a sony dvd-burner. 450 w dual fan power supply.
I am running windows XP pro.
When I run my machine at the 3000/400 it freezes up at sparatic times. If I run at 3000/333 it still freezes up but not as often. If I run at 2800/333 or lower it runs just fine.
I know it isn't heat related, the CPU temp is only 40 degrees. It isn't virus related as norton if fully updated. All mircrosoft updates and all drivers are updated.